1. Technical Field
This application is related to devices for sensing strain in materials, and more specifically, to devices and techniques for measuring dynamic strain in materials using a fiber optic sensor.
2. Related Technologies
Historically, strain has been measured using a resistance type strain gage or a semiconductor type strain gage. Both types measure the electrical resistance of the strain gage, which is a function of the applied strain.
Resistance-type strain gages typically include a grid of very fine wire or foil bonded to the backing or carrier matrix. The electrical resistance of the grid varies linearly with strain. The carrier matrix is bonded to the surface, force is applied, and the strain is found by measuring the change in resistance. These bonded resistance strain gages are inexpensive, robust, and suitable for low frequency or static strain measurements. However, in order to minimize electromagnetic interference, resistance type strain gage systems typically position the electronics very close to the strain sensor itself. Typical use of a Wheatstone bridge in the sensor makes multiplexing these gages difficult.
Semiconductor strain gages are more sensitive than the resistance strain gages, and are often used for dynamic strain measurements. However, they are more expensive, fragile, and are sensitive to temperature changes. They are also subject to electromagnetic interference, cannot operate remotely easily, and their multiplexing is difficult.
Fiber optic Bragg grating strain sensors are useful for very remote sensing applications, such as oil drilling. However, the detection electronics can be very expensive, and the strain detection limit of such a sensor is moderate, as discussed in A. D. Kersey, T. A. Berkoff, and W. W. Morey, “Multiplexed fiber Bragg grating strain-sensor system with a fiber Fabry-Perot wavelength filter”, Optics Letters, Vol. 18, p 1370-1372 (1993). Formation of Bragg gratings in optical fibers is discussed in G. Meltz, W. W. Morey, and W. H. Glenn “Formation of Bragg gratings in optical fibers by a transverse holographic method”, Opt. Lett., Vol. 14, p 823-825 (1989).
In the fiber Bragg sensors, the single mode fiber section with the Bragg grating written in it is glued to the sensing surface. Any change in the strain applied on the sensing surface along the direction of the fiber changes the Bragg grating periodicity which, in turn, changes the wavelength of the light reflected back by the Bragg grating. Thus, by detecting the reflected light wavelength modulation the applied strain can be detected. This reflected light wavelength modulation is the transduction mechanism of the sensor. Because even very long fiber leads do not introduce any reflected light wavelength modulation, these sensors are suitable for remote sensing. In addition, the sensor multiplexing capability is good since along the same fiber various different periodicity Bragg gratings can be utilized as different strain sensors. By using a broadband light source, light with different wavelengths is reflected from different Bragg gratings and can be detected separately. The Bragg grating sensor is ideal for very long distance strain sensing, such as in oil drilling. However, for most strain applications the detection electronics, which uses interferometric or non-interferometric schemes, is fairly complex and expensive.
Optical fiber has been used in other sensing applications, including microphone and microbend sensors, as disclosed in J. A. Bucaro and N. Lagakos, “Lightweight fiber optic microphones and accelerometers”, Rev. Scient. Instr., Vol. 72, pages 2816-2821 (2001); G. He and F. W. Cuomo, “Displacement Response, Detection Limit, and Dynamic Range of Fiber-Optic Lever Sensors”, J. Lightwave Technol. Vol 9, page 1618-1625; and in U.S. Pat. No. 7,020,354, U.S. Pat. No. 6,998,599, and U.S. Pat. No. 7,149,374.
Recently, fiber optic interferometric strain sensors have been introduced that are very sensitive, have a wide dynamic range, are immune to electromagnetic interference, can operate remotely, and can be multiplexed. Some interferometric sensors are described in E. Udd, Fiber Optic Sensors, p 271-323, 2006. Interferometric fiber optic strain sensors are discussed in Yuan et al., “Recent progress of white light interferometric fiberoptic strain sensing techniques”, Rev. Scient. Instr., Vol. 71, pages 4648-4654 (2000).
Current transduction mechanisms are phase, wavelength, or intensity modulation. In phase modulated interferometric sensors, the phase of the light propagated in the sensing fiber of the interferometer changes when an applied strain changes. Interferometric sensors with Mach-Zehnder or Michelson interferometers can detect extremely small strains by using long sensing fibers. However, due primarily to polarization effects, these sensors are complex and expensive and, thus, their use is generally limited to special applications of weak strain signals.
Some strain sensors rely on Fabry-Perot interferometry. The Fabry-Perot interferometer is formed by the end of an optical fiber and another surface parallel to the fiber end. Most of these sensors use a laser and a single mode fiber to enhance the coherence of the interferometer. These sensors have high sensitivity, and their detection scheme is easier than that of the Mach-Zehnder or Michelson interferometric sensors.